Cylinder has small wierd bumps when using Subdivision Surface
This is how subdivision works with faces that are created by that many vertices.
You'll see that on other parts of the cylinder, particularly the quads, that this effect does not happen. It's less of a problem, and more of just how the modifier works.
To fix this I suggest two things:

Delete only the face on top and extrude the edges left behind inward a couple times.




Keep the face on top, but create an edge crease along the top of the cylinder, (and use Auto Smooth if needed)




Creating an edge crease is done by hitting Shift + E, and you'll have to scroll your mouse out to get the full effect. The Auto Smooth is located in the Data tab and controls which parts of the mesh are shaded smooth or not depending on their angle respective to another/other face(s).